Cosmetics with diverse functions are proposed recently. For example, they include cosmetics capable of easily keeping the skin healthy, capable of favorably adhering to the skin and capable of being easily washed away, cosmetics containing ingredients capable of preventing aging and keratinization such as collagen, hyaluronic acid, squalane and urea or an ingredient capable of preventing skin roughening such as allantoin, skin whitening cosmetics containing an ultraviolet absorber such as benzophenone or zinc oxide for preventing blackening, ephelides or freckles, or containing a melanin production inhibitor such as arbutin or squalane, or capable of activating skin cells, cosmetics containing a moisture retaining agent or moistening agent such as glycerol, hyaluronic acid, silicone or lanolin and capable of keeping the skin moist, fresh and youthful, cosmetics containing an organic substance and capable of keeping the intended cosmetic effect lasting longer, cosmetics capable of preventing the darkening or partial glistening of the skin, cosmetics capable of expressing quality such as transparency or color tone, etc.
To impart these functions, various oily ingredients, moisture retaining agents, thickeners, whitening agents, ultraviolet absorbers, fine particles, dyes and the like for protecting the skin are mixed with water or any other solvent. These practices involve such production problems that it may be difficult to homogeneously disperse the respective ingredients and to stabilize the produced emulsions. Furthermore, the produced cosmetics are required to be good in the homogeneity and dispersibility of ingredients contained in them and furthermore to be excellent in long-term storage stability. Moreover, cosmetics are required to be excellent in the feeling of use during make-up, for example, in touch to the hand, smooth spreadability and touch to the skin, long-lasting in spite of the perspiration produced after make-up, and easy to remove.
Studies for solving the above-mentioned various problems of conventional cosmetics are conducted by using surfactants and natural dispersing agents, or loading such bases as inorganic fine particles, organic fine particles, polymer gels, natural gels and collagen with various compounding ingredients, or dispersing various compounding ingredients using acrylamide-based polymeric thickeners, etc., for improving the homogeneous dispersibility and stability of respective compounding ingredients.
In the recent studies of cosmetics, for dispersing an oil ingredient as a compounding ingredient, microdispersion techniques of using fine particles with a particle size of 1 μm or less as the oil ingredient are being studied (for example, JP10-147506A, JP2001-214081A, and JP2001-261526A). Furthermore, for using inorganic fine particles as a compounding ingredient, techniques of mixing fine particles with a diameter of 0.1 μm or less (hereinafter called nanoparticles) are being studied (for example, JP05-186323A, JP2000-264632A, JP07-002639A, JP2001-089314A, and JP2003-300844A). The homogeneous dispersion of an oil ingredient consisting of fine particles or a solid ingredient consisting of fine particles as described above can be improved to some extent by conventional methods such as selecting the lipid used or using an optimum surfactant for surface tension control. However, it is more difficult to achieve long-term storage stability when the diameter of the fine particles is smaller. Especially nanoparticles are highly likely to cohere to each other, and on the contrary to the intended dispersion, they form secondarily cohering particles of micron sizes, to settle. They have a problem that the intended object of homogeneously dispersing nanoparticles cannot be achieved.
For enhancing the homogeneous dispersibility of compounding ingredients and fine particles or for keeping the dispersed state stabilized for a long period of time, the use of a glycerol (for example, JP07-185294A and JP2000-128760A), the use of an acrylamide (for example, JP06-211626A and JP10-067685A), and the like are studied. However, these methods include cases where the dispersibility of the dispersing agent per se is not sufficient or where the long-term stability is not sufficient. For example, oil-in-water emulsions in which the diameter of acrylamide particles is 50 to 1000 nm (for example, JP10-087428A) are disclosed. However, this dispersing agent, if used as fine particles, makes the user feel sticky due to the nature of acrylamide per se, having a disadvantage that the freshness, refreshing feel and natural feel expected for the cosmetic used are lost, though it is good in smooth spreadability to the skin and excellent in touch to the skin when it is applied to the skin.
In this situation, demanded are materials good in the homogeneous dispersion of compounding ingredients and fine particles, long-term storage stability, adhesion to the skin and smooth spreadability, excellent also in touch to the skin, furthermore free from the sticky feel during use, and also excellent in the freshness, refreshing feel and natural feel expected for the cosmetic used.
As methods for obtaining such materials, it is proposed to use a clay material such as talc or bentonite or inorganic particles as a carrier and to let compounding ingredients adhere to it for dispersion. However, since the particle size of the carrier is as large as more than several micrometers, it is difficult to homogeneously disperse it into to the cosmetic, and since the particle size of the carrier is large, the user feels gritty, posing a problem of impairing freshness and natural feel.
As other methods, studied are cosmetics containing natural fibers such as collagen fibers as a compounding ingredient other than said organic fine particles and inorganic fine particles (for example, JP55-28947A, JP63-215770A and JP08-27192A). These cosmetics use materials modified to allow easy permeation or absorption into the skin by lowering the molecular weight of collagen or by chemically modifying collagen fibers, and though the materials are fibers, the configuration and function of the fibers used as a carrier are not so significant. Cases where silk fibroin fibers are made finer are also disclosed (for example, JP11-100510A). However, while they are short fibers with a length of 1 to 200 μm, they have a diameter of about 10 μm, and they should be called a silk powder of 10 μm or more in particle size rather than fine fibers. As particles, they are large, and the silk powder per se is poor in dispersibility and is likely to settle. These properties are not sufficient as the properties required as a material for carrying other nanoparticles to be dispersed in them. Moreover, there are further other methods in which cellulose fibers are used (for example, JP62-39507A), and in the case where such cellulose fibrils are used, the cellulose fibril fibers are very irregular in diameter, ranging from 1/10 to 1/100, consisting of large diameter fibers and small diameter fibers mixed together. It is very difficult to homogeneously disperse them, and furthermore the fibers also have a disadvantage that since the large diameter fibers are likely to settle, fine particles settle together rather than being dispersed. Moreover, the fibers have such disadvantages that mold and mildew are generated during storage and that the fibers per se are highly rigid and insufficiently flexible.
Furthermore, there are cases where cellulose nanofibers are used (for example, JP13-2523A), but the fibers have such problems that they are low in absolute strength, that the cellulose fibers are broken into fragments when dispersed, and that because of cellulose, mold and mildew are generated during storage of the dispersion. From this point of view, it is required to use ultrafine fibers made of a synthetic polymer, instead of cellulose.
As cosmetics containing ultrafine fibers made of a synthetic polymer, “cosmetics containing ultrafine fibers” intended for obtaining luster like velvet or natural luster like baby's lanugo (for example, JP2001-64153A) are disclosed. Though the ultrafine fibers used here are as short as 50 μm or less in fiber length, they have a fiber diameter of 2 μm (0.055 dtex). So, in the case where the fibers are mixed in a cosmetic, they are still large in fiber diameter, insufficient in flexibility and poor in affinity with the skin, making the user feel stress from the cosmetic coating, and can be used only for special make-up application. Furthermore, the fibers per se are insufficient in dispersibility into water or oil and in affinity with fine particles. So, though they can be used as ultrafine fibers for woven fabrics, knitted fabrics, nonwoven fabrics, etc., it is difficult to apply them in the cosmetic field, since they are insufficient in fiber diameter and flexibility.
In the meantime, methods for producing a synthetic paper from ultrafine fibers of a synthetic polymer are known, and various methods have been studied to use a dispersion of fibers for wet papermaking, etc. The number average diameter of ordinary single synthetic fibers is as large as 10 μm or more, and it is difficult to fibrillate them unlike natural pulp or cellulose. The fibers can be little entangled with each other, and it is difficult to obtain a synthetic paper with good evenness. So, for synthetic papers of polyester fibers, it was studied to use a binder together with polyester fibers for papermaking. The diameters of fibers used in these studies were about 13 μm (for example, JP49-8809B), about 15 μm (for example, JP55-110545A and JP60-34700A), and about 11 μm (for example, JP1-118700A). However, the synthetic papers obtained were rather insufficient in flexibility. Moreover, when the paper thickness was reduced for enhancing flexibility or air permeability, a synthetic paper with good evenness could not be obtained since the fibers were thick and poor in dispersibility. Furthermore, in the case where the paper thickness was forcibly reduced, the paper became irregular in the weight per unit area sometimes, not allowing practical use.
In this situation, recently synthetic papers composed of ultrafine fibers with a diameter of 10 μm or less are also being studied. As for the methods, the sea component is dissolved or physically removed for separation from an islands-in-sea multi-component fiber or from a splittable conjugate fiber, to prepare ultrafine fibers, and the obtained ultrafine fibers are used to produce a synthetic paper. The basic methods for producing such ultrafine fibers are already disclosed (for example, U.S. Pat. No. 3,382,305), and the ultrafine fibers per se are also disclosed (for example, U.S. Pat. No. 3,546,063). According to them, a method of removing the sea component from an islands-in-sea multi-component polyester fiber using an adequate solvent is used to obtain ultrafine fibers, and it is suggested that the ultrafine fibers can be used to produce a paper-like structure. However, since the ultrafine fibers obtained were very irregular in diameter, ranging from 0.01 to 3 μm, a practically usable synthetic paper was not obtained.
Thereafter, methods for treating an islands-in-sea multi-component fiber or a splittable conjugate fiber of 10 μm or less by a high pressure fluid for obtaining synthetic papers of ultrafine fibers (for example, JP56-169899A) are proposed. However, it was difficult to practically use the methods, for such reasons that it was difficult to uniformly fibrillate the fibers and that a special high pressure fluid device was necessary. Furthermore, islands-in-sea multi-component polyester fibers were dispersed and beaten in water, to obtain a synthetic paper composed of polyester fibers with a diameter of 1.5 to 4 μm (for example, JP4-10992A). Moreover, splittable conjugate fibers respectively consisting of polyolefin based resins different in components were beaten and the obtained fibers were used to produce a synthetic paper (separator material) (for example, JP2003-59482A). These fibers were about 5 μm in fiber diameter, and the split single fibers were uneven in form. So, they were very irregular in diameter. Furthermore, synthetic papers obtained by using the ultrafine fiber bundles of islands-in-sea multi-component fibers or splittable conjugate fibers and their short fibers were disclosed (for example, JP2003-253555A), but the fibers of the synthetic papers had a large diameter of 2 to 7 μm.
In addition, methods in which ultrafine fibers obtained by fibrillating liquid crystal fibers are used to obtain a synthetic paper are proposed (for example, JP8-209583A and JP2002-266281A). However, in these methods, though very fine fibers can be obtained by fibrillation, thick fibers not fibrillated so much also remain to mix with the very fine fibers. So, only a synthetic paper very irregular in single fiber diameter could be obtained.
On the other hand, in the applications of synthetic papers, especially in the fields of air cleaner filters, industrial dust removing filters, pure water producing filters, chemical reagent refining filters, medicinal/medical filters, battery separators, etc., a thinner synthetic paper with a uniform weight per unit area and a high strength is being demanded. The reason is that highly accurate control is required for removing very fine impurities outside the system or for recovering very necessary fine components in electronics field, mechatronics field, water quality field, drug/chemicals or food handling field, etc. So, there have been needs for studies on synthetic papers composed of nanofibers.
Methods of using conventional spinning techniques for islands-in-sea multi-component fibers allow the production of single fibers with a diameter of about 1 μm, but do not allow the production of fibers with a diameter smaller than it. Thus, the methods cannot sufficiently meet the needs for nanofibers. Furthermore, methods for obtaining ultrafine fibers from blended polymer fibers (for example, JP3-113082A and JP6-272114A), and the smallest diameter of the single fibers obtained as ultrafine fibers is about 0.4 μm. Thus, the methods cannot sufficiently meet the needs for nanofibers either. Moreover, the diameter of the single fibers obtained as ultrafine fibers is decided by the dispersion of the polymer used as the island component in the blended polymer fibers, and since the dispersion of the polymer used as the island component in such an ordinary polymer blend system is insufficient, the obtained ultrafine fibers are very irregular in single fiber diameter.
In the meantime, as a simple technique for reducing the diameter of ultrafine fibers to the nanometer level, a technique called electrospinning is spotlighted in recent years. The basic technique of the method has been known since a long time ago, and the method was proposed about 1935. The reasons why this technique is highlighted are that the nanofiber nonwoven fabric (like a synthetic paper) produced by this method is suitable especially as a material for cell culture in the biomedical field of USA, and that nonwoven fabrics of various polymers can be easily produced for research. In this method, a solution obtained by dissolving a polymer into an electrolyte solution is extruded from a die. In this technique, a high voltage of several thousand to thirty thousand volts is applied to the polymer solution, and the folding and expansion of the high speed jet and the subsequent jet of the polymer solution are used for forming ultrafine fibers. Usually these ultrafine fibers are bundled to be collected as a nonwoven fabric like a synthetic paper. If this technique is used, single fibers with a diameter of tens of nanometers can be obtained, and the diameter can be reduced to 1/10 or less of the diameters obtained by the conventional polymer blending techniques as the case may be. The polymers used are mostly biopolymers such as collagen and water soluble polymers, and in some cases a solution obtained by dissolving a thermoplastic polymer into an organic solvent may also be electrospun. However, each of the ultrafine fibers obtained even by this method often consists of ultrafine fiber portions connected by thick fiber portions (beads with a diameter of 0.5 μm), and each of the ultrafine fibers is very irregular in single fiber diameter {for example, Polymer, Vol. 43, 4403 (2002)}. Therefore, it is attempted to inhibit the production of the thick fiber portions for uniforming the fiber diameters, but the irregularity remains large, the problem remaining yet to be solved {for example, Polymer, Vol. 40, 4585 (1999)}. Furthermore, since the nonwoven fabric obtained by electrospinning is obtained as the solvent is evaporated in the step of fiber formation, the fiber aggregate is often not oriented or crystallized, and a nonwoven fabric with a strength very lower than those of ordinary nonwoven fabrics only can be obtained to greatly restrict its applicable range. Moreover, because of the solvent evaporated in the step of fiber formation, the electrospinning as a production technique has such problems that any measure must be taken to improve the working environment and that the solvent must be recovered. Furthermore, the nonwoven fabric that can be produced is also limited in size, and the size that can be produced is about 100 cm2. Moreover, the discharge rate is several grams per hour at the largest, to lower the productivity. In addition, a high voltage is necessary, and since a harmful organic solvent and ultrafine fibers float in air, risks of electric shock, explosion and poisoning keep lingering. So, the method has been practically difficult.
As described above, needed is a synthetic paper composed of nanofibers not limited in the selection of polymers, allowing a wide range of applications and small in the irregularity of single fiber diameter.
Meanwhile, the following formula (1) holds between the fineness (dtex) usually often used for the fibers described in the above-cited patent documents, etc. and the number average diameter φ (μm) of the single fibers used to form the synthetic paper of this invention.φ=10×(4×dtex/πp)1/2  (1)where dtex is the thickness of a fiber, at which the fiber with a length of 10000 m weighs 1 g (JIS L 0101) (1978).
For example, for converting a fineness into the number average single fiber diameter referred to in this invention, if the polymer is nylon, the number average diameter can be obtained from the following formula with the specific gravity as 1.14 (of nylon 6).φn6=10.6(dtex)1/2 If the polymer is not nylon 6, the specific gravity of the polymer can be used in the above formula for calculating the number average single fiber diameter.